Let me add a couple of thoughts to Mike's advice.
You actually should talk to a doctor before you fast. You want to make sure your system can handle it. Even if you're healthy, your body could have problems with fasting, depending on your normal diet. For example, if you eat a lot of carbohydrates . . . cereals with lots of sugar, breads, pasta, sweets . . . beside not being healthy, when you fast, your body reacts as it craves the sugars in the starch. So you will get cravings and you will experience problems with fatigue. Sugar cravings can cause you to become irritable or even angry.
There may also be medical issues you don't know about so talk to your doctor.
A condition called Hypoglycemia is fairly common. it's similar to diabetes but much less serious. A lot people don't know they have it. The body has problems processing sugar so it overreacts both when levels are lower or higher. Fasting might not be appropriate as well if you are on certain kinds of medication.
Now let's talk about the spiritual component. Fasting is a discipline that helps us to increase in the virtue of self-control. The idea is that if you learn to deny yourself, you learn to deny sin but, as with any Christian discipline, we must do it by grace through faith. St. Paul wrote in Romans Chapter 8 that we put to death the deeds of the flesh by the spirit. Otherwise it becomes our own work and that leads to the vice of pride. So when you fast, you need to be in prayer asking for the grace, needed and believing God will give you that grace because He is the giver of all good gifts to the point of giving Himself for us and to us. When tempted to break the fast, pray to God saying, I really want food, but I want you more.
You need to use wisdom. I know people that fast once a week for one day, drinking only water and you should drink water no matter what. I also know people who have periodically fasted on only water for much longer but it's not the kind of thing you can do all at once.
If your doctor approves, you may want to first try shorter fasts on a regular basis.
Maybe for a few weeks on certain day, skip one meal . . . then for one week on that same day only have one meal . . . and work up to it that way.
You see it's not about how long you fast. It's about learning how to rely on grace. We even learn by grace to rely on grace. You don't want this to be a goal to accomplish lest it become a source of pride and defeats the purpose of fasting. It's better to develop a consistent pattern of allowing God to teach you how to rely His Grace, than just to fast for two days. Eventually you may fast for two days but this assumes it's not unhealthy for you.
Fasting isn't an end . . . it's a means or a tool.